Conventionally, when a bar code in which bars are arranged in parallel in the scan direction (vertical bar code) is printed by a thermal printer, the bar code is printed by heating heat generating bodies arranged linearly at the position of the bars while the print medium is moved in a sub scan direction. The printing is performed by coloring a thermal paper using a thermal method or by melting and thermally transferring ink on an ink ribbon to a print medium using a thermal transfer method. FIG. 11 illustrates a conventional method for printing a bar code.
In FIG. 11, the right side illustrates a print medium P and a thermal head 2 equipped on a thermal printer, and the left side illustrates an enlargement of a print result. Heat generating elements 2a are linearly provided on the thermal head 2. The print medium P is carried in the sub scan direction, and a bar code B is printed on the left side of the print medium P. The bar code B is printed in parallel in the main scan direction by heating the corresponding heat generating elements on the thermal head 2 while moving the print medium P in the sub scan direction. When there is a space in the bar code B, a gap is obtained as a space by not printing a linear area, as shown by white circles on the left side. As conventional technology, Japanese Laid-Open Patent Application No. 2008-3994 discloses a device that prints such a bar code.
However, in the above-described conventional bar code printing, a so-called tailing phenomenon occurs, in which the dots stretch in the sub scan direction. This phenomenon is explained using FIG. 12, which is an explanatory diagram illustrating conventional bar code printing. The left side illustrates an ideal print result in the bar code printing, and the right side illustrates an actual print result by a thermal printer. In FIG. 12, the printing is performed by heating the heat generating elements inside a thermal head in a thermal printer. At this time, because the print medium is carried in the sub scan direction (direction of arrow A), the heat from the heat generating elements that were heated for printing the bars does not decrease even in spaces, causing the dots to stretch in the sub scan direction (direction to the space). This is the so-called trailing phenomenon. This trailing phenomenon has an advantage that that when a vertical frame is printed, the dots form vertical lines. However, when a bar code is printed, there is a problem that distortion occurs in a ratio of a bar width and a space width, and thereby a rank (e.g. important level) of the bar code drops or, in the worst case of scenario, the bar code becomes unreadable.
The present application has an object to provide a bar code printing device that realizes bar code printing in which the ratio of the bar width and the space width is made appropriate by preventing the trailing phenomenon at the time of printing the bar code.